1. Field of the Invention
The present invention relates to a photothermographic material.
2. Description of the Related Art
Recently, in the field of films for medical imaging, there is a strong demand for reducing the volume of waste processing liquid from the viewpoints of environmental preservation and economy of space. There have been demands for technologies relating to use of a photothermographic material as a film for medical imaging and a film for graphic arts. In particular, there is a demand for a photothermographic material that can be efficiently exposed by a laser image setter or a laser imager, and can provide black-toned images with high resolution and sharpness. Such a photothermographic material can provide users with a more simple and ecological thermal developing system without the use of liquid processing chemicals.
Although there are similar demands in the field of general image forming materials, high image quality (i.e., excellent sharpness and fine graininess) is particularly required for images used in medical imaging where high image quality of excellent sharpness and granularity are necessary. Further, images with blue-black tones are preferred from the perspective of facilitating diagnosis. Various types of hard copy systems using pigment or dye, such as an inkjet printer and an electrophotograph system, are commonly used as a general image forming system. However none of these is satisfactory as an output system for medical images.
In general, thermal image forming systems using organic silver salts are described in U.S. Pat. Nos. 3,152,904 and 3,457,075 and on pages 279 to 291, Chapter 9, “Thermally Processed Silver Systems,” written by D. Klosterboer, in (Imaging Processes and Materials) Neblette, 8th edition, compiled by J. Sturge, V. Walworth and A. Shepp (1989), the disclosures of which are incorporated herein by reference.
A photothermographic material typically includes a photosensitive layer in which a photocatalyst (e.g., silver halide) of a catalytically active amount, a reducing agent, reducible silver salt (e.g., organic silver salt) and a toner for controlling the tone of a developed silver image as needed are dispersed in the matrix of a binder. After an image is exposed thereon, the photothermographic material is heated to a high temperature (e.g., 80° C. or above) to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (which acts as an oxidizing agent) and a reducing agent, thus providing a black silver image. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image of the exposed silver halide. As a result the black silver image is formed in the exposed region (see U.S. Pat. No. 2,910,377 and Japanese Patent Application Publication (JP-B) No. 43-4924). Further, Fuji medical dry imager FM-DP L is an example of a practical medical image forming system using a photothermographic material that has been marketed.
In production of a photothermographic system including an organic silver salt, two methods are available. In one method, a solvent coating is adopted, and in the other method, a coating liquid containing polymer fine particles as a main binder in an aqueous dispersion is applied and dried. In the latter method, since no necessity arises for a process of solvent recovery or the like, a production facility is simple and the method is advantageous for mass production.
Clear images have been obtained through intensive improvements such as forming the image forming layer using a binder mainly comprising a hydrophobic latex for excluding the effect of moisture on the photographic properties in the thermal imaging system (see for example Japanese Patent Application Laid-open (JP-A) No. 2002-171936) having an image forming layer applied as an aqueous coating and taking advantage of organic silver salts.
Stable images may be formed without depending on preservation conditions by forming at least two layers including a protective layer at the image forming layer side in either of the application methods using a solvent or aqueous solution. While photosensitive materials having good resistance to scratches and good conveyability in handling before and during processing of the photosensitive material may be obtained, the at least two layers are preferably applied at the same time considering the production cost.
When a polymer derived from an animal protein (for example gelatin) is used at the outermost layer (see for example JP-A No. 2002-162712), water resistance is not sufficient, and time-dependent variation of sensitivity is not sufficiently stable when the photosensitive material is preserved under high temperature and high humidity conditions. While a dye that serves as a background color is often used in order to control the tone of the image obtained, it was found to be another problem that the color is transferred when the images are preserved in layers.
Since the outermost layer is hydrophilic, the layer becomes sticky, while being poor in image stability due to fingerprints left behind.
While a layer excellent in water resistance may be formed by applying a coating solution mainly comprising latex (for example JP-A Nos. 2000-227643 and 2001-194744), it was revealed that it is quite difficult to uniformly apply the coating layer without forming an irregular surface since the layer cannot be set after coating, and stability of the coating solution at the outermost layer is particularly sensitive to a salt concentration of the adjacent layer, to pH of the adjacent layer, or to the surface charge of added dispersed particles in the adjacent layer, such that the surface of the photosensitive material tends to be wrinkled in the drying step after coating.
Since the coating solution containing the polymer latex usually has a low viscosity, dispersed matting agent particles added for improving conveyability tend to be precipitated after mixing. Moreover, the dispersed particles are readily aggregated, resulting in the problem of rapid precipitation. Image quality may be decreased and handling performance such as conveyability may be deteriorated when precipitation of the dispersed particles is remarkable.
Accordingly, in the photothermographic material formed using the aqueous coating, the surface state of the outermost layer is quite important with respect to the influence on thermal developing apparatus, efficiency of the coating step, image properties and the like.